November 27, 2022

Are they more likely in September? What does the science say?

Read Time:5 Minute, 11 Second


(CNN Spanish) — And it happened again. As if it were something that only happens in fiction, Mexico was hit once again by a strong earthquake in September.

What makes this fact more incredible is that this Monday’s earthquake took place on September 19, a date well remembered in Mexico for the occurrence of earthquakes in previous years.

The bitter memory of this date began with the magnitude 8.1 earthquake of September 19, 1985, which affected the center, south and west of the country, leaving approximately 10,000 dead. This earthquake was the one that led to the creation of the National Civil Protection System of Mexico.

This was followed by the earthquake of September 19, 2017. Due to the events of 1985, a mega drill is carried out in the country on this day as a preventive measure; the one in 2017 was around noon. Approximately one hour after the drill, central Mexico was hit by a magnitude 7.1 earthquake.

Now, 32 and 5 years after the earthquakes of 1985 and 2017, respectively, another earthquake greater than magnitude 7 occurs in the country. On this occasion, it also happened almost an hour after the macrodrill; its magnitude was 7.6 according to the United States Geological Survey (USGS) and 7.7 according to the National Seismological Service (SSN) of Mexico, which is part of the National Autonomous University of Mexico. Mexico (UNAM).

And, to increase the incredibleness of the situation, there have been other earthquakes in September that lead to the same bad memory due to the proximity to the 19th: on September 7, 2017, prior to the anniversary of 1985 and the new earthquake, an earthquake of magnitude 8.2 occurred that greatly affected the southwest of the country; and four years later, on September 7, 2021another was recorded, now of magnitude 7.1, with an epicenter on the coast of Acapulco.

The tremors that have marked Mexico in September 0:51

So, does the probability of earthquakes increase in September in Mexico? What does science say?

Due to the occurrence of several large earthquakes in September and on similar dates (and even at similar times for the last two of the 19th), doubts have been raised in Mexico about whether earthquakes occur more often in this month.

And it’s a reasonable doubt. Who would have thought that this situation would occur so precisely? It seems very incredible. But science is categorical about this: it is a coincidence.

Víctor Hugo Espíndola Castro, an investigator for the SSN, said at a press conference this Monday that it is nothing more than “an unpleasant coincidence”.

“This earthquake (on September 19, 2022) occurred at 1:05 p.m. (local time). There will be those who say that the one in 2017 also occurred at the same time (it was at 1:14 p.m.). Also remember that it does one year there was one on September 7 in the Acapulco region, of magnitude 7.1. But it is nothing more than a coincidence, an unpleasant coincidence,” said Espíndola Castro.

How likely is the occurrence of large earthquakes on the same date in a given year? The probability is very low, the expert added, but it is not impossible. Therefore, when we talk about probability, we refer to the chances in numbers of something happening, and even if the figures are low, there may be a scenario in which an event becomes a reality.

“If we look at the entire seismic catalog that we have since the early 1900s, we see that earthquakes greater than 7 have been repeated in different months. For example, we see several in the month of December, several in all months. So, certainly the The probability of it occurring on the same day is very low, but just because a probability is small does not mean that it cannot occur.

“(The probability of) an earthquake occurring again on September 19 may be very small, but it is not impossible. In other words, the probabilities only tell us about that, about what is more likely to happen,” Espíndola explained.

To make the matter of probability clear, the SSN investigator mentioned that the coincidence of the earthquakes on September 19 “is like winning the lottery three times in a row, or once, if you like, every four years.” In short, there is a low probability of it happening, but it is not something impossible.

Likewise, he added some data that also come from probabilities: for example, that statistically every three years an earthquake greater than magnitude 7 occurs on the Mexican coast, or that every six years there is one of magnitude 8.

“That is a forecast, but basically it is still statistical and that does not give us much information to use those techniques to predict, which is very different,” said Espíndola Castro.

For his part, Luis Quintanar Robles, academic secretary of the UNAM Institute of Geophysics, said at the conference that until now there is no scientific certainty that ensures that earthquakes in Mexico occur on a specific date.

“What we can say is that earthquakes generally trigger when there is a release of stress produced in a neighboring area. So, if we consider that, for example, on September 14 there was a magnitude 5 earthquake in the Guerrero area, It is possible that this earthquake has released some efforts that have facilitated the movement between the oceanic and North American tectonic plates,” which could have resulted in Monday’s earthquake, he explained.

However, this tells us that the force of a previous earthquake could help rub against surrounding plates to trigger another earthquake, but not that tremors occur on the same day in September.

“Obviously, at this time the only thing I can say is that it is a coincidence. However, this question is open (…) and I believe that it is possible to study the possible origin of what is called a birthday of an earthquake, that they occur on the same day of the year. But at this time we cannot say anything in a convincing way about it, “said Quintanar Robles.



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